Limb skeletal malformations – What the HOX is going on?

Synpolydactyly (SPD) is a rare congenital limb disorder caused by mutations in the HOXD13 gene, a homeobox transcription factor crucial for autopod development. The hallmarks of SPD are the webbing between the third and the fourth finger and the fourth and the fifth toe, with a partial or complete digit duplication in the syndactylous web. Different classes of HOXD13 mutations are involved in the pathogenesis of synpolydactyly, but an unequivocal genotype–phenotype correlation cannot always be achieved due to the lack of structure–function data of HOXD13. Mutations in DNA binding or polyalanine tract domains of HOXD13 result in predictable clinical outcomes. However, mutations outside of these domains cause a broad variety of clinical features that complicate the differential diagnosis. In this review, we summarize the different classes of HOXD13 mutations causing synpolydactyly phenotypes with respect to their underlying pathogenic mechanism of action. In addition, we emphasize the importance of the chicken embryo as an animal model system for the study of (limb) development and potential genotype–phenotype correlations in SPD or other human malformation syndromes.